According to U.S. Pat. No. 3,293,292 it is essential for the preparation of acetic acid to use both manganese and cobalt (e.g., in their 2+ form acetate tetrahydrates) with a source of bromine (e.g., ammonium bromide) to oxidize butane with oxygen gas in the liquid phase at 176.degree.-177.degree. C. and a gauge pressure of 65.4 kg/cm.sup.2 in the presence of acetic acid as reaction solvent.
More recently U.S. Pat. No. 4,111,986 discloses that acetic acid can be prepared by contacting a sufficient concentration of oxygen-containing gas (e.g., oxygen gas at at least 5 liters per hour per 100 grams of butane) with normal liquid butane in the presence of an acetic acid solution of components of catalysis consisting essentially of cobalt (e.g., 1 to 50 milliequivalents per mole of butane) and bromine (2 to 500 milliequivalents per mole of butane). For this process reaction temperatures of at least 176.degree.-177.degree. C. are preferred at gauge pressures of from 35 up to 211 kg/cm.sup.2, preferably from 56 up to 105.5 kg/cm.sup.2.
Concentrated acetic acid (even glacial) distilled from the effluent produced by the foregoing liquid phase oxidation processes is contaminated with bromine-containing compounds, and is not generally suitable as an article of commerce even though the commercial specifications for glacial acetic acid or acetic anhydride do not set a maximum allowable value for a bromine concentration.
Also acetic acid becomes contaminated with bromides when used as solvent or reaction medium for the liquid phase oxidation of alkyl-substituted aromatic compounds (e.g., xylenes, toluene, trimethyl benzenes) with air to the corresponding aromatic carboxylic acid in the presence of catalysis provided by the components comprising a combination of one or more transition metal oxidation metal catalysts and a source of bromine (e.g., Br.sub.2, HBr, inorganic bromide salt, an organic bromide such as tetrabromoethane). While some who practice such process for the production of aromatic carboxylic acids reuse the bromide contaminated acetic acid in the alkyl aromatic oxidation process, others (e.g., the assignee of U.S. Pat. No. 3,578,706) prefer to remove the bromine or bromine-containing contaminants before reusing the acetic acid in the oxidation process.
According to said U.S. Pat. No. 3,578,706 the bromine contaminated acetic acid is treated by reaction with a metal having electrochemical potential between manganese and iron, inclusive and then contacting the acetic acid with an anion exchanger to remove the bromine or bromides.
Such bromine contaminated acetic acids can contain both ionic and coordinate forms of bromine (e.g., bromine attached to carbon) which are not entirely removed by distillation or fractionation but rather carry through to the 97-100% acetic acid fraction in amounts of from 0.0005 up to 0.015 weight percent total of said two forms of bromine. We have found that by a simple two step process the concentrated acetic acid can be purified to a bromine content below the present capability of analytical detectability which is, on a weight basis, 3 parts bromine per 1.times.10.sup.6 parts (i.e., 3 ppm) acetic acid.